The star of TV show Charmed who came to Perth for "Oz Comic Con" has posted photographs of herself on social media with Sea Shepherd representatives, wearing one of their beanies.

Omar Todd, from Sea Shepherd, tweeted on Sunday: "How to sell out really quickly of @SeaShepherd beanies? Well when @DohertyShannen wears one they disappear fast! #outofstock!"

Doherty then tweeted another photograph of herself wearing the beanie, telling Adelaide fans who will see her next: "Free hug if you wear one!!"

For her forthcoming 43rd birthday, Doherty has even asked fans to donate to Sea Shepherd, rather than give her gifts at forthcoming conventions.

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Sea Shepherd Australia is a non-profit conservation organisation which aims to end the destruction of habitat and the slaughter of wildlife in the ocean.

Under WA's shark cull trial, any bull, tiger or great white shark bigger than three metres caught on baited drumlines set one kilometre off the metropolitan and southwest coast would be be shot dead and dumped further out at sea.

Drumlines have so far caught at least 104 sharks, including 101 tiger sharks, with 40 either found dead or destroyed, figures released earlier this month show.

WA's Environmental Protection Authority has declined to formally assess the program, despite receiving a record number of public submissions.

Sea Shepherd also recently failed in its bid to secure a Supreme Court injunction to force the state government to suspend the cull. The trial runs until April 30.